1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus, and in particular to an apparatus for measuring blood vessel diameter, blood flow velocity, or the like.
2. Description of the Related Art
Ultrasonic diagnostic apparatuses are used for diagnosing blood vessel condition, heart functionality, and the like. When measuring displacement of a blood vessel wall using an ultrasonic diagnostic apparatus, the blood vessel wall is automatically tracked on the path of an ultrasonic beam passing across the blood vessel based on the echo data. Displacement as time passes of a blood vessel wall is in synchronism with a heart beat. Observation of the heart beat in this manner can provide fundamental data for use in diagnosing diseases, such as heart failure, arterial sclerosis, and so on.
For measurement of the speed of a bloodstream within a blood vessel using an ultrasonic diagnostic apparatus, a sample gate is securely provided on the path of an ultrasonic beam within a blood vessel, and Doppler information can be extracted from echo data concerning the inside of the sample data. Based on the extracted Doppler information, a blood velocity (an averaged-speed within the sample gate) is calculated. The resultant data on a blood velocity is fundamental data useful in diagnosing the condition of the heart and blood vessels.
Conventional ultrasonic diagnostic apparatuses measure displacement of a blood vessel wall and a blood velocity in different measurement modes.
Meanwhile, a new evaluation value, wave intensity, has been established as a diagnostic index. Originally, wave intensity was proposed as an index for determining which of the functions of forward pulse waves and backward pulse waves is dominant, the forward pulse wave being a pulse wave traveling from the heart to periphery, the backward pulse wave being a pulse wave returning from periphery to the heart. Specifically, wave intensity is defined as follows.
I=xcex94Pxc2x7xcex94Uxe2x80x83xe2x80x83(1) 
wherein P is blood pressure at a local part within an artery, U is blood velocity at the local part, and AP and AU are respective changes in P and U during a period xcex94t.
That is, wave intensity is defined as the product of changes in pressure P and blood velocity U during a constant short time period xcex94t. The magnitude of xe2x80x9cIxe2x80x9d depends on as the definition of xcex94t. Meanwhile, time-normalized wave intensity, which has the same property as that of the above xe2x80x9cIxe2x80x9d and is normalized with respect to time, can be expressed as follows.
WI=(dP/dt)xc2x7(dU/dt)xe2x80x83xe2x80x83(2) 
In the equation (2), wave intensity WI is defined as the product of time differential of blood pressure P and that of blood velocity U.
One method proposed for measuring wave intensity includes a noninvasive measurement method using ultrasonic. In this method, an ultrasonic echo tracking method and an ultrasonic Doppler method are combined.
Specifically, in order to measure, for example, wave intensity in the carotid artery of a subject, conventionally, an ultrasonic probe which comprises one transducer (first transmitter/receiver) for blood vessel wall measurement and one transmission transducer and two beam receiver transducer for Doppler measurement (second transmitter/receiver) is abutted on the cervical part of the subject. Then, ultrasonic pulses are transmitted to receive echo using the first transmitter/receiver, so that the wall position of the carotid artery is automatically tracked based on the echo data. Based on the tracking, a change of a blood vessel diameter is measured. Meanwhile, ultrasonic pulses are successively transmitted to receive echoes using the second transmitter/receiver, and Doppler information is extracted from the echo data to be analyzed. Based on the analysis, a change as time passes of blood velocity is measured.
A close correlation between a change of a blood vessel diameter and that of blood pressure has conventionally been understood. Therefore, a change waveform concerning a blood vessel diameter can be regarded as a change waveform concerning blood pressure by considering the largest and smallest blood vessel diameters respectively as the maximum and minimum blood pressure values, which are measured using a cuff-type hemodynamoneter applied to the upper arm of the subject.
Further, wave intensity can be obtained in an off-line calculation using the above equation (2) based on a change of blood velocity and that of blood pressure.
In the above conventional method, however, the tomogram of a blood vessel cannot be displayed as the first transmitter/receiver comprises a single transducer for an A mode. This makes it impossible to visually confirm that an ultrasonic beam is passing across the center of a blood vessel, which in turn can lead to problems with the reliability of measurement. Moreover, while a sample point for Doppler information, which is a point where the transmission beam and two reception beams formed by the second transmitter/receiver intersect with one another, is fixedly positioned, it is uncertain whether or not the sample point falls on the center of a blood vessel. When a sample point be set close to, or on, an interior wall of a blood vessel, or even in the outside of a blood vessel, measurement accuracy is significantly deteriorated. That is, measurement reliability can not be guaranteed.
In a general view, no conventional ultrasonic diagnostic apparatus can simultaneously display a tomogram, a waveform concerning displacement of a blood vessel wall (or a blood vessel diameter), and a blood velocity waveform. In addition, no conventional ultrasonic diagnostic apparatus has a function for automatic real-time measurement of wave intensity.
Here, in order to measure displacement of a blood vessel wall and a blood velocity, the direction of an ultrasonic beam relative to the blood vessel wall or a bloodstream must be known. However, conventionally, there is a problem that it is difficult to set an ultrasonic beam intersecting with a blood vessel wall or a bloodstream at a predetermined angle. Moreover, there is another problem in simultaneous measurement of displacement of a blood vessel wall and a blood velocity, that it is difficult to set separate beam directions preferable to the respective measurements.
The present invention has been conceived in light of the above situation, and with an object of improvement of reliability in ultrasonic measurement of a tissue such as a blood vessel.
Another object of the present invention is achievement of highly accurate simultaneous measurement of blood velocity and change of a blood vessel diameter.
Still another object of the present invention is simultaneous, real-time display of a plurality of measured information concerning a blood vessels and on the like, so that comprehensive diagnosis of such can be achieved.
In order to achieve the above objects, an ultrasonic diagnostic apparatus according to the present invention transmits an ultrasonic pulse and obtains echo data in respective measurements of displacement of a blood vessel wall and of a blood velocity. Then, a tomogram of a blood vessel is prepared based on the echo data, and a measurement line relative to the blood vessel axis is automatically or manually set in the tomogram. When the position of a blood vessel wall on the measurement line is specified, the specified position is tracked so as to calculate displacement of the blood vessel wall. In addition, using the measurement line as a reference, a sample gate is set within the blood vessel, for use in extraction of Doppler information. Using the Doppler information extracted from echo data, the speed of a bloodstream flowing in the sample gate is calculated. Then, an evaluation value is calculated based on the displacement of the blood vessel wall and the blood velocity.
With the above arrangement, an ultrasonic pulse is transmitted in the respective measurements of displacement of a blood vessel wall and of a blood velocity. That is, according to a pulse Doppler method, a range resolution can be obtained, and a sample gate can be freely set within a blood vessel for extraction of Doppler information. Generally, transmission of a broadband ultrasonic pulse for measuring displacement of a blood vessel wall and that of a narrow band ultrasonic pulse for measuring blood velocity are separately executed in a time sharing manner in various possibly set pulse transmission patterns.
Various evaluation values can be calculated, with the most preferable of these being the wave intensity which is described by the above equations (1) or (2). The evaluation values may be used as parameters for other calculations.
According to another aspect of the present invention, the ultrasonic diagnosis apparatus calculates a blood vessel diameter based on the position of the blood vessel wall specified on the measurement line. Using as a reference the input maximum and minimum blood pressure values, a change of the blood vessel diameter is converted into a change of blood pressure. Then, using the thus calculated blood pressure and the blood velocity at the sample gate, calculated using the Doppler information contained in the echo data, an evaluation value is calculated.
In this arrangement, the diameter of a blood vessel is converted into blood pressure (pressure at a focused part in a blood vessel) according to the maximum and minimum blood pressure values (or a blood pressure signal) input. That is, blood pressure is estimated from the diameter of a blood vessel utilizing conventionally known knowledge concerning strong relationship between a change of the diameter of a blood vessel and a change of blood pressure.
Desirably, the evaluation value calculator for calculation of an evaluation value may include means for calculating a time differential of the blood pressure, means for calculating a time differential of the blood velocity, and means for calculating wave intensity based on the time differentials of the blood pressure and of the blood velocity.
According to still another aspect of the present invention, a time differential of the speed of a bloodstream flowing in a measurement part within a blood vessel, which is obtained based on the echo data, is obtained. Further, a time differential of the blood pressure at the measurement part, which is obtained based on the echo data and/or a bio-signal, is also obtained. Then, the time differentials of the blood velocity and of the blood pressure at the same moment are multiplied by each other to thereby calculate wave intensity.
According to yet another aspect of the present invention, the ultrasonic diagnostic apparatus sets a beam direction passing across a blood vessel, and transmits an ultrasonic pulse in the beam direction to obtain echo data in that beam direction. Based on the echo data concerning that beam direction, the positions of the anterior and posterior walls of the blood vessel are specified, and a blood vessel diameter is calculated based on the specified positions of the anterior and posterior walls of the blood vessel.
Change of the blood vessel diameter over time is converted into change of blood pressure value over time, and the pressure is used in at least one of image formation and data calculation.
This arrangement makes it possible to obtain blood pressure information which can not be obtained using a conventional ultrasonic diagnosis apparatus. Therefore, various operations or imaging using a blood pressure value are achievable. For example, the magnitude of blood pressure may be indicated by means of color-coding the blood vessel tomogram.
According to yet another aspect of the present invention, the ultrasonic diagnosis apparatus prepares a blood velocity graph showing change over time of the velocity of blood flowing in a blood vessel based on echo data, and a blood vessel diameter graph showing a change over time of a blood vessel diameter based on the echo data. The apparatus then calculates an evaluation value from the blood velocity and the blood vessel diameter obtained at the same moment, and prepares an evaluation value graph showing a change as time passes of the evaluation value. The blood velocity graph, the blood vessel diameter graph, and the evaluation value graph are simultaneously displayed.
According to yet another aspect of the present invention, the ultrasonic diagnostic apparatus prepares a tomogram of a blood vessel based on echo data. The apparatus also prepares a blood velocity graph showing change over time of the velocity of blood flowing in the blood vessel based on the echo data, and a blood vessel diameter graph showing change over time of a blood vessel diameter based on the echo data. The tomogram of the blood vessel, the blood velocity graph, and the blood vessel diameter graph are simultaneously displayed.
According to yet another aspect of the present invention, the ultrasonic diagnostic apparatus transmits and receives an ultrasonic pulse for ultrasonic beam scanning to obtain a received signal. Based on the received signal, a first beam direction is determined so as to be orthogonal to the blood vessel wall, and set to the transmitter-receiver. Then, displacement of a blood vessel wall is measured using the received signal corresponding to the first beam direction, and an evaluation value is calculated using the displacement of the blood vessel wall. This arrangement allows the transmitter-receiver to control the direction of an ultrasonic beam (a position of a beam axial line) transmitted and received by a probe. As a result, the first beam direction can be automatically set so as to be orthogonal to the blood vessel wall based on the received signal.